This invention relates to a receiver for improving radio delay accuracy and an associated method.
In many wireless communications applications it is necessary to make accurate estimates of received signal delay for radio receivers. For, example, in wireless communications systems which use time difference of arrival (TDOA) measurements it is critical that an accurate estimate of the received signal delay can be determined. Such wireless communications systems include systems for mobile location tracking, ranging receivers such as radar, and systems where the propagation delay of the radio receiver must be known.
One application of mobile location tracking is Enhanced 911 (E-911) service, which includes the location of the emergency caller so that the emergency response to the mobile caller may be timely. An emergency caller calling from a cellular phone in an automobile may not be aware of his exact location, and may be too upset to determine that location. Mobile location tracking allows the location of the callers cellular phone (and the caller) to be determined, and the emergency caller may then be found that much more quickly.
The accuracy of mobile location tracking for E-911 services is important not only from a practical point of view, but also has been mandated by the FCC. Specifically, the FCC has mandated (Docket No. CC94-102) that all U.S. wireless providers must have the ability to locate an emergency mobile within 125 meter accuracy with 67% confidence by Oct. 1, 2001.
In addition to E-911 services, applications such as navigation services, roadside assistance, cargo tracking, and home zone-billing would benefit from mobile location tracking. Potentially, these other services could be built on top of an E-911 system.
In general, there are two approaches to mobile location tracking, a receiver network based approach, and a handset based solution. One advantage of a receiver network based approach over a handset based approach is that a receiver network based approach supports mobile handsets already in the market. The present invention takes a receiver network based approach.
Time difference of arrival (TDOA) measurements are one way of determining the location of a mobile transmitter (such as a cell phone) for mobile location tracking applications using a network based approach. For a TDOA measurement, the position of a mobile transmitter is determined by examining the difference in time at which a signal from the transmitter arrives at receivers in multiple base stations. Assuming the signal propagates in free-space, the range, Ri, from the transmitter to an ith receiver will be c times t0i, where t0i is the time required for the signal to travel in a line from the transmitter to the ith receiver at the speed of light, c. Similarly, the range, Rj, from the transmitter to the jth receiver is t0j times c. For a given TDOA measurement for two particular receivers, the transmitter must lie on a hyperboloid with a constant range difference, Rij=Rixe2x88x92Rj, of the two receivers given by the following equation (1):
Ri,j={square root over ((Xi+L xe2x88x92x)2+L +(Yi+L xe2x88x92y)2+L +(Zi+L xe2x88x92z)2+L )}xe2x88x92{square root over ((Xj+L xe2x88x92x)2+L +(Yj+L xe2x88x92y)2+L +(Zj+L xe2x88x92z)2+L )}
where (x y z), (Xi Yi Zi), and (Xj Yj Zj) are the location coordinates of the transmitter, receiver i, and receiver j, respectively. The receiver coordinates are known and the transmitter coordinates may be determined using equation 1 if a sufficient number of independent TDOA measurements are performed. If the receivers are in the same plane (z is equal to a known constant), at least three receivers are required to determine the mobile transmitter location because two equations are required to solve for the x and y position variables. If the transmitter and receivers are not in the same plane, at least four independent TDOA measurements are required to fix the location of the transmitter which means that at least four base stations, each with a receiver are required.
FIG. 1 shows a two-dimensional hyperbolic position location solution where the transmitter and receivers are assumed to be in the same plane. In the network system of FIG. 1, three receivers, S1, S2, and S3, receive a signal from a transmitter, T. The relative range of the transmitter to the receivers S1 and S3 is R13. The relative range of the transmitter to the receivers S1 and S2 is R12. The hyperboloids for the relative ranges, R13 and R12 are shown in FIG. 1 and are labeled as R13 and R12, respectively. The intersection of the hyperboloids is the location of the transmitter.
For a TDOA measurement the absolute time that a signal requires to travel from the transmitter to the receivers is not critical. Instead, TDOA measurements are based upon the difference in the time required for a signal to travel between the transmitter and one receiver from the time required to travel between the transmitter and another receiver. Because the difference in the time for the signal to travel must be known accurately, TDOA measurements require that the receivers have precisely synchronized clocks. Often atomic clocks such as a Cesium time source are used.
Additionally, accurate TDOA measurements require that any time delay due to the receiver hardware be accurately accounted for. The measured difference in time, tij, that a signal from a transmitter requires to travel to receiver i compared with the travel time to receiver j (the time of arrival (TOA)) is given by the equation:
tij=(t0ixe2x88x92t0j)+(tRixe2x88x92tRj)xe2x80x83xe2x80x83(2)
where t0i and t0j are the time for a signal to travel from the transmitter to receiver i and receiver j, respectively, and tRi and tRj are the delay time due to the receiver hardware of receivers i and j, respectively. Ideally, all the receivers involved in a TDOA measurement would have the same hardware delay time and the contribution to the measured difference in time, tij, from the hardware would be zero, i.e., tRixe2x88x92tRj=0. Alternatively, if the hardware delay time of each receiver were different, but fixed and known, the difference in delay time between receivers could be readily compensated for.
All radio frequency (RF) analog components contribute to the group time delay incurred by the mobile transmitter signal until it is digitized. Wide band receiver hardware elements such as amplifiers have short stable delays. The delays due to these wide band hardware elements are either negligible or may be readily accounted for. Filters, on the other hand, will have a larger delay, which varies from unit to unit, and over time and temperature. Consequently, the group time delay profile at different receivers becomes intractable, which may significantly impact locating performance. A particularly difficult problem is the first intermediate frequency (IF) filter employed in a heterodyne receiver chain. This filter is typically fairly high order to provide image rejection. Typical technologies for these filters are surface acoustic wave (SAW) and crystal filters. These filters may exhibit asymmetry with respect to the IF frequency and this asymmetry may vary with temperature and time.
One attempt to solve the problem with IF filters requires extensive calibration and correction for IF filter response and variation with temperature based on manufacturing measurements. This is time consuming and does not account for variation in IF filter behavior with time. Another approach is to increase the specs on the IF filter. However, this drives up product cost and may provide less IF rejection.
It is an object of one embodiment of this invention to provide a method for compensating for the asymmetrical group time delay of an IF filter in a radio receiver for TDOA measurements by performing two time of arrival (TOA) measurements. In the first TOA measurement the time delay is measured when the receiver uses a RF LO frequency equal to the center RF frequency of the signal received by the receiver minus the IF frequency, and in the second TOA measurement the receiver uses a RF LO frequency equal to the center RF frequency plus the IF frequency. The asymmetry of the IF filter is compensated for by using the two TOA measurements. In one aspect of this embodiment the time delay of the two TOA measurements is averaged. This method easily compensates for IF filter asymmetry which may vary with temperature, or over time because consecutive signals received by the receiver from a transmitter may be used for the two TOA measurements.
It is another object of one embodiment of this invention to provide a method of compensating for the group time delay of an IF filter in a radio receiver for TOA measurements which neither requires an expensive IF filter with high specs, or extensive calibration of an IF filter to determine its group time delay response.
It is another object of another embodiment of this invention to provide for a radio receiver which may compensate for the asymmetrical group time delay of an IF filter by including a switch in the radio receiver which switches between a first RF LO frequency and a second RF LO frequency. The switch may automatically switch between the first and second RF LO frequencies for consecutive signal bursts received. The receiver includes a mixer which downconverts a signal in the radio receiver according to the frequency of the local oscillator. The receiver may also include a demodulator and a TOA estimator connected to the demodulator and to the switch for receiving a demodulated signal from the demodulator and for causing as the switch to switch between the first frequency and the second frequency of the local oscillator.
It is another object of another embodiment of this invention to provide a method for compensating for the group time delay asymmetry of a component of a receiver in a TOA measurement. In this embodiment a first signal is downconverted to a first converted frequency signal by a first oscillator frequency, and a second signal is also downconverted to a second converted frequency signal. Both the first converted frequency signal and the second converted frequency signal are passed through the component with the group time delay asymmetry. A TOA measurement is performed for both the first and second signals, and the group time delay of the component is compensated for using the first and second TOA measurements. For example, the group time delay may be compensated for by averaging the group time delay for the first and second TOA measurements.
It is another object of yet another embodiment of this invention to provide for a receiver which may compensate for the asymmetrical group time delay of the component in the receiver by including a switch in the receiver which may switch between two frequencies of the oscillator. The receiver includes a mixer which downconverts a signal in the receiver according to the frequency of the oscillator. The component transforms the downconverted signal to a converted signal, and the component exhibits group time delay asymmetry. The first oscillator frequency is equal to the center frequency of the signal received by the receiver minus the frequency of the downconverted signal. The second oscillator frequency is equal to the center frequency plus the frequency of the downconverted signal. The switch preferably automatically switches between the first and second oscillator frequencies for consecutive signal bursts received. The receiver may also include a TOA estimator connected to the component and to the switch for receiving the converted signal from the component and for causing the switch to switch between the first and second frequencies of the oscillator.
It is another object of yet another embodiment of this invention to provide for a receiver network for mobile location tracking having receivers and a central processing center for receiving time of arrival values from the receivers, where each receiver may compensate for the asymmetrical group time delay of an IF filter.